In optical transceivers used for long-distance transmission, the dynamic range of optical receivers is relatively set to be wide in order to cope with both short-distance and long-distance optical transmissions. For example, in an optical transceiver for 100-gigabit Ethernet (for example, 100GBASE-ER4 conforming to the Institute of Electrical and Electronic Engineers (IEEE) standard) applicable to 40-kilometer transmission, the optical modulation amplitude (OMA) receivable by the optical receiver is set to −21.4 dBm to +4.5 dBm in order to cope with transmission distance from 0 to 40 kilometers.
In such an optical receiver, a semiconductor optical amplifier (SOA) amplifies an optical signal to meet a wide dynamic range. The SOA outputs the amplified optical signal to a receiver optical sub assembly (ROSA) having a light receiving element built in. Specifically, the SOA amplifies the optical signal such that the amplified optical signal falls within an optical power range (dynamic range) in which the amplified optical signal can be received by the ROSA with an allowable error rate or no error. That is, the SOA amplifies the optical signal with a high amplification factor (high gain) when the optical input power of the optical signal received from the outside is relatively small, and amplifies the optical signal with a low amplification factor (low gain) when the optical input power of the received optical signal is relatively large. The amplification allows the amplified optical signal to fall within the dynamic range of ROSA. For example, Japanese Unexamined Patent Publication No. 2011-172202 and Japanese Unexamined Patent Publication No. 2010-136195 disclose techniques for controlling a current (SOA current) for driving an SOA to adjust a gain (SOA gain) of the SOA and to cause optical input power to an ROSA to fall within the dynamic range of the ROSA.